Structure-Property Relationship of Thermoset Nanocomposites

Abstract

In this thesis we report the synthesis, characterization and thermo-mechanical properties of a high-temperature resistant themoset nanocomposite system based on an aero-space-grade Bismaleimide resin. Various processing techniques with various fillers are used. The emphasis is on establishing the relationship between the structure and mechanical properties of nanocomposite systems. We characterized the nanocomposite systems experimentally using rheology, X-ray diffraction, Thermo-mechanical and microscopic techniques. The mechanical properties e.g. viscoelastic properties are interpreted in terms of the microstructure and explained by using micromechanical and viscoelastic models. In order to get insight into the structure of clay particles in the form of suspension we studied the rheology of organoclay dispersions before curing. We investigated the development of organoclay dispersions over time with the help of rheometry by applying small amplitude oscillatory deformation. Dispersions evolve over time with distinct stages into a percolating network. In most of the cases with various clay concentrations the behavior of dispersions was elastic solids-like. There is a critical threshold concentration of clay particles at which the dispersions initially behave as elastic solids and below which they form viscous fluids. This critical threshold seems to coincide with overlap concentration of the bodies of revolution of the particles, which is at a low clay concentration (of the order of 0.5% w/w). This overlapping of the bodies of revolution of particles may also limit the degree of exfoliation. Complete exfoliation is hardly ever achieved, as usually the concentration of particles used is much larger than the critical threshold concentration. Moreover, surprisingly, the frequency dependency of the mechanical moduli of the dispersions resemble that of a critical gel (a system just at the cross over between a visco-elastic solid and a visco-elastic fluid), normally reported for cross-linking polymers. This aspect has not been highlighted yet for clay dispersions. Interestingly the critical gel-like behavior of the dispersions persisted throughout the evolution over time. Thermo-mechanical properties of nanocomposite systems prepared with both carbon nanofiber and organoclay were investigated. The matrix itself and the nanocomposite system show excellent thermal properties and reasonable mechanical properties, better than the normal engineering polymers. The use of carbon nanofiber did not produce significant improvement in mechanical properties due to the poor adhesion of the fiber with the matrix. However, the use of organo clays shows systematic increase in mechanical properties and heat deflection temperature with the concentration of clay particles. The evaporation of solvent during curing leads to alignment of clay particles, which may also be beneficial for the properties of the nanocomposite. The stiffness of the nanocomposite was modeled by the Halpin-Tsai model. The model reproduces the data reasonably well. XRD results and the apparent aspect ratio obtained by Halpin-Tsai fitting indicate that the nanocomposite system is not completely exfoliated, and that the degree of exfoliation decreases with increasing particle concentration. We also investigated creep behavior of the nanocomposite system. The matrix shows very good creep stability and the use of nanofiller further enhances it. Application of the Findley power law and the Burgers model, which are widely used to describe the creep behavior of polymers, is critically evaluated. Their limitations to describe the creep behavior of thermoset matrices are discussed. We used a modified form of Burgers’ model which we named the ‘stretched Burgers model’ (SB) to describe the creep behavior of thermoset matrix and the nanocomposite. The stretched Burgers model reproduces the time-dependent creep compliance remarkably well. We made assumptions in fitting the data that retardation time scale distribution should be independent of filler concentration. The very good fitting of data supports the assumption. This means that the dynamics of the nanocomposite system is mainly governed by the dynamics of the matrix. This is an interesting assumption in our study and never highlighted in creep studies of nanocomposites. We believe that this finding is helpful for developing a better understanding of the mechanics of nanocomposites and of the role of filler on the dynamics of the matrix, which is greatly debated. The stretched Burgers model appears to be very suitable for describing the creep behavior of thermoset systems both from a physical point of view and concerning the quality of the fits.